In many applications, for example for driving fans for refrigeration systems, or pumps for domestic electric appliances, there is currently a tendency to replace the traditional asynchronous electric motors with electric motors of synchronous type having permanent magnets.
Synchronous electric motors of this type have better performance, with consequent energy saving, a greater constructional simplicity and lower weight, and consequently are of reduced cost.
Permanent magnet single phase synchronous motors have, however, starting problems due to the fact that the rotor has to be brought from stationary to rotation at a speed in general tied to the frequency of the alternating voltage of the supply network. If the supply source is the normal mains distribution network at 50 at 60 Hz the rotor of such a motor has to reach the synchronism speed in a half-period. This is particularly problematic when the starting resistance torque is not zero, and therefore the load which must be driven to rotate has a high inertia as in the case of an axial fan.
A further problem of single-phase synchronous motors is that the initial direction of rotation of the rotor is in general not pre-determined, that is to the say the motor is per se, not of a unidirectional starting type.
A further problematic aspect of the synchronous motor is represented by the fact that its speed of rotation under normal running conditions is strictly tied to the frequency of the supply voltage.